The present invention relates to a laminated ferrimagnetic thin film having a magnetization reversion characteristic suitable for a free magnetic layer of a magneto-resistive effect element or a ferromagnetic tunnel junction element, and a magneto-resistive effect element and a ferromagnetic tunnel element using this thin film.
The magneto-resistive effect is a phenomenon that an electric resistance varies based on an external magnetic field. A magneto-resistive effect element senses a magnitude of the external magnetic field applied to the element as a change in an element resistance value, or has a function to hold this state as a magnitude of the element resistance value. In particular, there are many expectations to application of a spin valve type GMR element and a ferromagnetic tunnel junction element each having a large magneto-resistive change ratio (MR ratio) in a low external magnetic field area at a room temperature to a high recording density compatible reproduction magnetic head and a high density solid-state magnetic memory (MRAM).
These magneto-resistive effect elements consist of a free magnetic layer capable of setting a magnetization direction free with respect to the external magnetic field, a pinned magnetic layer including a mechanism to fix a magnetization direction to the external magnetic field, and a non-magnetic layer which is arranged between and comes into contact with these layers.
The magneto-resistive effect occurs when a resistance of the element varies due to a relative magnetization angle of the magnetic layers on the both sides with the non-magnetic layer sandwiched therebetween. Substantially eliminating the magnetic coupling between the free magnetic layer and the pinned magnetic layer realizes a so-called spin valve type element operation that only a magnetization direction of the free magnetic layer varies with respect to the external magnetic field, and obtains the magneto-resistive effect sensitive to the external magnetic field.
As magnetic materials (ferromagnetic substances), there are widely known Co, Ni, Fe and alloys of these materials. In particular, Co or an alloy including Co, especially a CoFe alloy has a feature of large saturation magnetization, and Ni or an alloy including Ni, especially an NiFe alloy has a feature of a excellent soft magnetic characteristic (low coercive force). As free magnetic layer materials of the magneto-resistive effect element, a magnetically soft (superior in a soft magnetic characteristic) magnetic material like NiFe is suitable and widely used because of the need to quickly perform magnetization reversion in a small external magnetic field.
Further, as to a mechanism which fixes a magnetization direction of the pinned magnetic layer to an external magnetic field, there are employed a method (coercive force difference type) which uses a magnetic material which is magnetically hard as a material and makes its reverting coercive force larger than that of the free magnetic layer, and a method (exchange biased type) which further positively fixes the magnetization direction to one direction by bringing an anti-ferromagnetic layer into contact with a pinned magnetic layer.
The element which uses a non-magnetic conductor such as Cu, Ag and Au as a non-magnetic layer is a spin valve type GMR element typically disclosed in U.S. Pat. No. 5,159,513, and an element which uses a non-magnetic insulator such as an Al oxide or an Al nitride as a non-magnetic layer is a ferromagnetic tunnel junction element typically disclosed in U.S. Pat. No. 5,650,958. A higher MR ratio can be obtained in the latter element rather than the former element. Furthermore, the both elements has a difference in direction along which an electric current is passed. Since the in-plane high integration of the element is important for an application to a high density solid-state magnetic memory (MRAM), the ferromagnetic tunnel junction element which has a higher MR ratio and performs energization in a direction vertical to the plane is more preferable.
On the other hand, as one of thin film structures which can be applied to the ferromagnetic layer (the free magnetic layer or the pinned magnetic layer) in these magneto-resistive effect elements, there is a multi-layer magnetic film which is called a laminated ferrimagnetic thin film. As apparent from the basic structure disclosed in U.S. Pat. No. 5,341,118, this is a multi-layer thin film consisting of at least two ferromagnetic layers and a non-magnetic intermediate layer which is arranged between and in contact with the ferromagnetic layers. When a non-magnetic transition metal which is preferable as a non-magnetic intermediate layer material is used, the extremely strong anti-ferromagnetic interaction (exchange bonding) acts on the both ferromagnetic layers through the non-magnetic intermediate layer, and their magnetization directions are always anti-parallel in a low magnetic field. Therefore, the magnetic moments of the respective ferromagnetic layers constituting the laminated ferrimagnetic thin film are always canceled out, and it is possible to realize an artificial ferrimagnetic state having a small magnetic moment corresponding to a difference in film thickness between the respective ferromagnetic layers. In the laminated ferrimagnetic thin film, the magnetic film thickness can be effectively greatly reduced without extremely decreasing the film thickness of each ferromagnetic layer.
The magnitude of the anti-ferromagnetic exchange bonding between the ferromagnetic layers in the laminated ferrimagnetic thin film depends on a material and a film thickness of the non-magnetic intermediate layer. Physical Review Letters, Vol. 67, p. 3598 (1991) describes that the intensity of the anti-ferromagnetic exchange bonding between the ferromagnetic layers in the laminated ferrimagnetic thin film depends on a non-magnetic intermediate layer material and the periodic vibrations are demonstrated with respect to a film thickness of the non-magnetic intermediate layer material. It also mentions that, in the laminated ferrimagnetic thin film which has a simple sandwich structure that the number of times of repetition=1 and in which film thicknesses of the respective ferromagnetic layers are equal to each other, assuming that HS is a saturation magnetic field, MS is saturation magnetization of the ferromagnetic layer to be used and t is a film thickness of the ferromagnetic layer, the bonding intensity (J: anti-ferromagnetic exchange bonding energy) can be obtained based on J=HS×MS×t/2. In this report, there is described the relationship between the anti-ferromagnetic exchange bonding intensity and many non-magnetic intermediate layer materials when Co is used as the ferromagnetic layer, as well as the tendency that the magnitude of the ferromagnetic exchange bonding energy increases in the order of a 5d group transition metal, a 4d group transition metal and a 3d group transition metal as the non-magnetic intermediate layer material and that it also increases as a quantity of outermost shell d electrons becomes larger in the same cycle. Among others, Ru, Rh, Ir and Cu have the anti-ferromagnetic exchange bonding energy of up to 10−3 J/m2 (1 erg/cm2), which is considerably larger than that of any other element. It is to be noted that, according to Journal of Magnetism and Magnetic Materials, Vol. 165, p. 524 (1997), assuming that t1 and t2 are the respective ferromagnetic layer film thicknesses, the anti-ferromagnetic exchange bonding energy when the respective ferromagnetic layers have different film thicknesses can be given by J=HS×MS×t1×t2/(t1+t2).
It is known that the exchange bonding intensity of the laminated ferrimagnetic layer also depends on a material for the ferromagnetic layer to be used. Physical Review B, Vol. 56, p. 7819 (1997) mentions that the anti-ferromagnetic exchange bonding energy when Cu is used as the non-magnetic intermediate layer and NiFe is used as the ferromagnetic layer is not more than J=6×10−5 J/m2 (0.06 erg/cm2), but this value is far smaller than that obtained when Co is used as the ferromagnetic layer. Further, Journal of Applied Physics, Vol. 73, p. 5986 (1993) mentions that the exchange bonding intensity becomes smaller when NiFe (perm alloy) is used as the ferromagnetic layer rather than Co, and points out the possibility of the influence of the fact that the saturation magnetization of NiFe is smaller than that of Co. Japanese patent application laid-open No. 143223/2001 discloses a film thickness range of the non-magnetic intermediate layers (Ru, Cr, Ir and Rh) by which the magnitude of a saturation magnetic field to be obtained is preferable when NiFe and Co are used as the ferromagnetic layer material in the laminated ferrimagnetic thin film. By using the relationship between the saturation magnetic field disclosed herein and the non-magnetic intermediate layer film thickness, the relationship between the saturation magnetization of each ferromagnetic layer material and its film thickness, and a relational expression of the anti-ferromagnetic exchange bonding energy J and the saturation magnetic field HS mentioned above (J=HS×MS×t1×t2/(t1+t2)), the non-magnetic intermediate layer film thickness dependence of each anti-ferromagnetic exchange bonding intensity when the ferromagnetic layer materials are NiFe and Co can be obtained.
FIGS. 1 and 2 are graphs showing the relationship between the anti-ferromagnetic exchange bonding energy of NiFe and Co obtained by the above method and film thicknesses of the non-magnetic intermediate layers (Ru, Rh, Ir).
Description will now be given as to a prior art concerning the application of the laminated ferrimagnetic thin film to the magneto-resistive effect element. The magneto-resistive effect element having a laminated ferri type pinned magnetic layer will be first explained. In the magneto-resistive effect element, as described above, in order to subject the free magnetic layer to magnetization reversion in accordance with a change in an external magnetic field (in order to cause the magneto-resistive effect element to operate in the spin valve manner), the magnetic effect acting between the free magnetic layer and the pinned magnetic layer must be reduced to substantially zero. When the laminated ferrimagnetic thin film is applied to the pinned magnetic layer, however, the influence of the pinned magnetic layer on magnetization reversion of the free magnetic layer can be further reduced. The basic structure of the magneto-resistive effect element having the laminated ferri type pinned magnetic layer is disclosed in U.S. Pat. No. 5,465,185. Usually, in the magneto-resistive effect element, the phenomenon that a fixed amount of the reverting magnetic field of the free magnetic layer is shifted can be observed due to the influence of the static magnetic field formed by the magnetic charge of an end of the pinned magnetic layer. In the magneto-resistive effect element having the laminated ferri type pinned magnetic layer, however, the static magnetic field formed by the pinned magnetic layer can be minimized by the magnetic thin film reducing effect of the pinned magnetic layer, thereby reducing the shift of the reverting magnetic field of the free magnetic layer. In the magneto-resistive effect element having the laminated ferri type pinned magnetic layer, an improvement to make the pinned magnetic layer more preferable (the magnetic effect of the pinned magnetic layer acting on the free magnetic layer is further reduced) has been advanced. For example, Japanese patent application laid-open No. 052317/2001 discloses a method by which grain growth is suppressed due to the buffer effect by realizing the three-layer structure with an appropriate material and film thickness of a ferromagnetic layer on a free magnetic layer side of two ferromagnetic layers constituting a laminated ferri type pinned magnetic layer, thereby smoothing the top face of the pinned magnetic layer on the free magnetic layer side. As a result, the direct ferromagnetic coupling (which is generally called nail coupling) due to roughness acting between the pinned magnetic layer and the free magnetic layer can be reduced. In such a laminated ferri type pinned magnetic layer, it is know that there is also an advantage to make the pinned magnetic layer magnetically more hard as well as the above-described advantages. That is, in the magneto-resistive effect element having the laminated ferri type pinned magnetic layer, since the pinned magnetic layer is hard to be reverted with respect to the external magnetic field, it is further preferable as the pinned magnetic layer whose magnetization direction must be fixed. Journal of Applied Physics, Vol. 85, p. 5276 reports the coercive force difference type ferromagnetic tunnel junction.
The magneto-resistive effect element having the laminated ferrimagnetic layer will now be described. Its basic structure is disclosed in U.S. Pat. No. 5,408,377. The advantage of using the laminated ferrimagnetic thin film for the free magnetic layer lies in that reducing an effective magnetic film thickness of the free magnetic layer can decrease the magnitude of a demagnetizing field formed by the free magnetic layer itself which is in proportion to the magnetic film thickness. The reverting magnetic field of the micro-fabricated free magnetic layer is increased in inverse proportion to an element size due to the effect of the demagnetizing field, but the magneto-resistive effect element using the laminated ferri type free magnetic layer can reduce the reverting magnetic field. As a result, in a magnetic head, the magnetic field sensitivity of a fine element can be improved. Also, in a solid-state magnetic memory, the switching magnetic field of the fine element can be reduced, thereby enabling power saving.
Thus, realizing minuteness of an element size is required with respect to a demand for the high recording density of a reproduction magnetic head and a demand for the high integration of a solid-state magnetic memory, and it is expected that suppression of an increase in the reverting magnetic field of the free magnetic layer caused due to the above-described realization of minuteness becomes very important in future. As different from the pinned magnetic layer, however, there is almost no improvement report about the magneto-resistive effect element in which the laminated ferrimagnetic thin film is applied with respect to the free magnetic layer.
However, in the prior art laminated ferrimagnetic thin film, the large anti-ferromagnetic exchange bonding energy can be readily obtained when Co or CoFe is used as a ferromagnetic layer material, but it is difficult to produce the large anti-ferromagnetic exchange bonding energy by using Ni or NiFe. Referring to FIG. 2, when Co is used as a ferromagnetic layer material, the large anti-ferromagnetic exchange bonding energy can be obtained in a large non-magnetic intermediate layer thin film range of approximately 0.6 nm if the non-magnetic intermediate layer is formed of Ru, Ir or Rh. However, referring to FIG. 1, the non-magnetic intermediate layer film thickness range in which the large anti-ferromagnetic exchange bonding energy of not less than, e.g., 10−3 J/m2 (1 erg/cm2) when NiFe is used does not exist if the non-magnetic intermediate layer is formed of Ru or Ir. Further, in case of Rh, this range is not more than 0.1 nm, which is very narrow. Namely, when Ni or NiFe is used as a ferromagnetic layer material, very precise control over the non-magnetic intermediate layer film thickness is required as compared with the case that Co or CoFe is used. When applying to a device, irregularities in film thickness between lots or in a wafer must be taken into consideration, which results in a serious problem. As the laminated ferrimagnetic thin film suitable for the free magnetic layer in the magneto-resistive effect element, there is required the laminated ferrimagnetic thin film which can simultaneously realize the large anti-ferromagnetic exchange bonding energy and the excellent soft magnetic characteristic equivalent to that of Ni or NiFe and does not require precise control of the non-magnetic intermediate layer film thickness.